Wave Spring for a Spinal Implant

ABSTRACT

A spinal implant includes a coiled wave spring configured to surround a nucleus. The wave spring is formed with at least one wire having a sinusoidal shape and made of a shape memory material. The shape memory material is tailored to achieve a stress-induced martensitic transformation when a critical stress is exceeded. The implant may further include an artificial nucleus configured to simulate a disc nucleus. Methods of forming and implanting the spinal implant are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/536,109 filed Jun. 28, 2012, now U.S. Pat. No. 9,039,766,which claims priority to U.S. Provisional Patent Application No.61/503,076 filed Jun. 30, 2011, the disclosures of which areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to medical devices, and more particularlyto spinal implants.

BACKGROUND ART

The vertebral column, also called the backbone, is made up of 33vertebrae that are separated by spongy discs and classified into fiveareas: (1) the cervical vertebrae which consists of seven bony parts inthe neck; (2) the thoracic vertebrae which consists of 12 bony parts inthe back area; (3) the lumbar vertebrae which consists of five bonysegments in the lower back area; (4) the sacrum which consists of fivesacral bones fused into one; and (5) the coccyx which consists of fourcoccygeal bones fused together into one. The five areas of the spine areshown in FIG. 1.

Lumbar disc disease occurs in the lumbar area of the spine. The lumbararea of the spine (and other areas of the spine) is made up of twoparts:

-   -   vertebral bodies—the parts that are made of bone.    -   intervertebral discs—also known as simply as discs. The discs        are located between the bony parts of the spine and act as        “shock absorbers” for the spine.

FIG. 2 shows an example of an intervertebral disc located between twovertebral bodies. The vertebral bodies are numbered from 1 to 5 in thelumbar area and the discs that are located between two of the vertebralbodies are numbered accordingly (e.g., L2-3, (or the disc locatedbetween vertebral bodies 2 and 3)).

Each intervertebral disc is composed of two parts: (1) the annulusfibrosis—a tough outer ring of fibrous tissue, and (2) nucleuspulposus—located inside the annulus fibrosis. The nucleus is composed ofa more gelatinous or soft material.

As humans age, the intervertebral disc may become dehydrated andcompressed. This condition leads to the deterioration of the tough outerring. FIG. 3 shows a healthy disc and a degenerated disc. As the toughouter ring degenerates, in some cases, the nucleus is pushed against thering and eventually bulges out of the ring. This is considered a“bulging disc”. As the disc continues to degenerate, and with continuedstress on the spine, the inner nucleus pulposus may actually rupture outfrom the annulus. This is considered a ruptured, or herniated, disc.FIG. 4 a shows an example of healthy disc, and FIG. 4 b shows an exampleof a herniated disc. The fragments of disc material can then press onthe nerve roots that are located just behind the disc space. This cancause pain, weakness, numbness, or changes in sensation. Most discherniations happen in the lower lumbar area, particularly in the L4-5and L5-S1 areas. FIG. 5 shows a herniated disc, while FIG. 6 shows anexample of a healthy disc. Note the uneven distribution of forces on theherniated disc.

Lumbar disc disease is due to a change in the structure of the normaldisc. Most times, disc disease is the result of aging and thedegeneration that occurs within the disc. Occasionally, severe traumacan cause a normal disc to herniate and trauma may cause an alreadyherniated disc to worsen.

Spinal fusion is a surgery that fuses vertebrae together. Typically, twovertebrae are permanently coupled so that there is no longer anymovement between them. In some cases, the surgeon will use a graft (suchas bone) to hold (or fuse) the bones together permanently. There areseveral different ways of fusing vertebrae together. In one example,strips of bone graft material may be placed over the back part of thespine to fuse two vertebrae together. In another example, the bone graftmaterial is placed between the vertebrae. In yet another example, aspecial cage is placed between the vertebrae and the cage is filled withbone graft material. In further examples, the vertebrae are fusedtogether using screws, plates, and/or cages.

There are many disadvantages associated with spinal fusion. Spinalfusion is designed to eliminate the normal motion of one or more lumbarsegments in the spine. Accordingly, the spinal column above and belowthe fusion area is more likely to be stressed when the spine does move.Thus, persistent stress can cause future problems in un-fused areas ofthe spine.

Disc nucleus replacement is a procedure that replaces the soft jellycenter of the natural disc (or a portion thereof) with a prosthetic disknucleus (PDN) such as an artificial gel sac. The gel sac alleviates painand further damage by acting as a shock absorber that prevents the spinefrom applying pressure to the nerves. Another potential benefit of thegel space is that it allows more movement of the spine, and thereforeprevents disk degeneration below and above the site of surgery. As aresult, the gel sac allows the cartilage surrounding the nucleus to healand the patient can resume normal activity. Disc nucleus replacementsurgery can be performed using a minimally invasive laparoscopicprocedure, which is performed through tiny cuts using miniature toolsand viewing devices.

One example of an in-situ curable polyurethane nucleus replacementdevice is the DASCOR™ Disc Arthroplasty device. The DASCOR™ device ismade by mixing two-parts of liquid polymer while delivering it through acatheter to an expandable polyurethane balloon that is placed in thedisc space. The polymer cures in a matter of minutes, changing statefrom a liquid to a firm, but pliable solid device. After 15 minutes, thedelivery catheter is removed, leaving the final implant device. Theballoon catheter has a low profile and can be inserted into the discspace through a small annulotomy (e.g., 5.5 mm). The mixed liquidpolymer is delivered to the balloon under controlled pressure, causingthe balloon to expand to contour and fill the entire disc space left bythe nucleotomy procedure.

The use of the in-situ curable device provides for implantation of alarge volume device through a small annulotomy, thus making migration ofthe solidified device unlikely. Additionally, the system has theversatility of creating an implant of whatever size that is created bythe nucleotomy.

Also, the device can be used in combination with other components suchas endplates that are affixed to the vertebrae. In particular, thedeployment of a large and pliable device located between the endplatesand that contours to the endplates can help balance associated loadtransfer between the annulus and the artificial nucleus while minimizingendplate disruption.

Another related advantage of the in-situ curable device is its abilityto generate distraction forces inside the nucleotomy space. Therefore,the implantation of the device not only offers the ability to fill anygiven space left by nucleotomy, but also the potential to distract andrestore a collapsed intervertebral disc.

A disadvantage associated with the in-situ curable device is that thepolymer might not be robust enough over time to support the compressiveloads of the spine.

Total disc replacement is another example of a spinal surgery. In somecases, the entire disc is beyond repair and a complete disc replacementis necessary. In such an instance, total disc replacement can beperformed instead of spinal fusion surgery. Nonetheless, total discreplacement has not yet been shown to be superior to spinal fusion.Total disc replacement involves replacing the disc with an artificialdisc. Some artificial discs (such as ProDisc, Link, SB Charite) consistof two metal plates and a soft core.

The SBCharité III is an example of an artificial disc used to replace anentire disc. The SBCharité III is composed of two endplates of highquality cobalt chromium alloy. The endplates are coated with titaniumand a hydroxyapatite porous coating to enhance bone fixation(osteointegration). The endplates are fixed to each vertebrae usinganchoring teeth along the edges of the plates. The natural movement ofthe disc is made possible with a dense polyethylene sliding core that isplaced between the endplates. In this manner, the core acts as a spacerto maintain a natural distance between the two vertebrae and also morenaturally supports the spinal column.

Unlike spinal fusion, disc replacement technologies (such as the DASCOR™and the SBCharité III) do not require grafts and provide for a morenatural movement of the spine so that further injuries to the spine arediminished. To this end, disc replacement technologies attempt torestore and maintain normal physiological motion. This is accomplishedby (1) restoring and maintaining a natural intervertebral separationheight, (2) restoring and maintaining a natural lordosis, (3) restoringand maintaining a natural instantaneous axis of rotation; (4) correctingabnormal motion; (5) reducing or eliminating pain in the spine, and (6)improving functional ability of the patient. If these goals areachieved, the segments of the spine adjacent to the artificial disc willbe free of abnormal loads and motions. Accordingly, there would be adeceleration or elimination of stress applied to spine segments adjacentthe artificial disc.

There are some disadvantageous associated with current disc replacementtechnologies. Artificial discs that use polymer materials tend todegenerate because polymer strength diminishes over time, especiallyunder loads, a phenomenon known as creep. As the polymer materialsdegenerate, the core between the endplates of the artificial disc willwear thin, changing the intervertebral distance and causing wear debristo undesirably migrate into the spinal area. The patient may react tothis debris with an inflammatory response that can cause pain, implantloosening, and further implant failure.

The artificial disc device itself may also be a source of complications.The device can shift out of its normal position and even dislocate. Ifthe device migrates out of position, it can cause injury to the nearbytissues. A second surgery may be needed to align or replace the device.Similar to other types of joint replacements, the artificial disc devicemay fail over time as its components degenerate. An artificial discdevice is estimated to last 15 to 20 years. Once the device fails, it isremoved and typically replaced with spinal fusion surgery.

Subsidence is another possible problem of artificial disc devices.Subsidence happens when the disc device sinks down into the vertebralbody or is pushed up into the vertebral body. Subsidence can result in aloss of the normal disc height, which, in turn, could result in thecompression of nerves and adverse neurological symptoms.

SUMMARY OF THE EMBODIMENTS

Illustrative embodiments of the present invention are directed to aspinal implant that includes a wave spring configured to surround anucleus. Other illustrative embodiments are directed to a method offorming a spinal implant that includes forming a spring having a shapeof a wave spring, and configuring the spring to surrounding a nucleus.Other illustrative embodiments are directed to a method of implanting aspinal implant that includes inserting a wave spring into anintervertebral space, and introducing a nucleus material into aninterior area of the spring. The nucleus material is configured tosimulate a natural disc nucleus.

In various illustrative embodiments, the spinal implant may furtherinclude an artificial nucleus configured to simulate a disc nucleus, andthe spring surrounds the artificial nucleus. The artificial nucleus maybe made from a polymer material, a hydro-gel material, and/or a wavespring. The spring may be wedge shaped. The spring may be made of ashape memory material, stainless steel, titanium, titanium alloy, and/ora cobalt chrome alloy. The shape memory material may be Nitinol and/or aTitanium-Niobium alloy. The spring may be formed with one or more flatwires and/or rectangular wires.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of embodiments will be more readily understood byreference to the following detailed description, taken with reference tothe accompanying drawings, in which:

FIG. 1 shows the five areas of the spine;

FIG. 2 shows an example of an intervertebral disc located between twovertebral bodies;

FIG. 3 shows a healthy disc and a degenerated disc;

FIGS. 4 a and 4 b show examples of a healthy disc and a herniated disc,respectively;

FIG. 5 shows another example of a herniated disc with the unevendistribution of forces on the disc;

FIG. 6 shows another example of a healthy disc with the distribution offorces on the disc;

FIG. 7 shows a spinal implant with a spring wave and a nucleus locatedwithin the spring, in accordance with one embodiment of the presentinvention;

FIG. 8 shows a spinal implant implanted within the space between theintervertebral space in accordance with one embodiment of the presentinvention;

FIG. 9 shows a wave spring in accordance with one embodiment of thepresent invention;

FIGS. 10 a and 10 b show a conventional coil spring and a wave spring,respectively, in accordance with one embodiment of the presentinvention;

FIG. 11 shows the stress-strain curve of Nitinol (NiTi), bone, andstainless steel in accordance with one embodiment of the presentinvention;

FIG. 12 shows the tensile behavior of stainless steel and Nitinol inaccordance with one embodiment of the present invention;

FIG. 13 a shows one portion of a wave spring manufactured using flatwires and FIG. 13 b shows a wave spring manufactured using flat wires inaccordance with one embodiment of the present invention;

FIG. 14 shows a stress-strain curve for a superelastic shape memorymaterial in accordance with one embodiment of the present invention; and

FIGS. 15A-15F shows a method for inserting a spinal implant inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the present invention are directed to aspinal implant 10 that includes a spring 12, such as a wave spring,configured for insertion into an intervertebral space, for example, asshown in FIGS. 7 and 8. In various illustrative embodiments, the spring12 has a controlled stiffness, offers many degrees of motion, providesfor the shock absorption of the vertebrae, acts as a spacer between thevertebrae, may support an inner artificial nucleus pulpous and/or helpsto prevent stress shielding of the vertebrae. In further exemplaryembodiments, the spring 12 is loaded and tuned to have the same modulusas bone and may be formed from a shape memory material.

In various illustrative embodiments, the spinal implant 10 may furtherinclude an artificial nucleus 14 which is designed to simulate thefunction of the natural disc nucleus. The spring 12 is configured tosurround the natural disc nucleus or the artificial nucleus 14, which ispreferably formed of a compressible material such as a compressiblepolymer and/or hydro-gel material. Additionally, or alternatively, theartificial nucleus 14 can be made from a spring, for example a wavespring. The spring can be designed to have mechanical properties similarto the native, natural disc nucleus.

The spring 12 forms an interior area 16 that is configured to hold theartificial nucleus 14. FIG. 7 shows a spinal implant 10 with a wavespring 12 and a nucleus 14 located within the spring 12, in accordancewith one embodiment of the present invention. In such an embodiment, thespring is used as a fiber annulus and is intended to facilitate acontrolled range of motion of the spine. In various illustrativeembodiments, the spring is “tuned” as explained above to match themodulus of the vertebral bone. In additional or alternative embodiments,the spring 12 will help to restore the distance between the vertebrae.In yet further embodiments, the spring 12 will support the compressiveand torsional loads on an artificial polymer nucleus 14, while alsobeing elastic so that stress shielding is minimized.

In further illustrative embodiments, the spring is compressed tofacilitate a minimally invasive implantation. FIG. 8 shows a wave springimplanted within the space between the vertebrae in accordance with oneembodiment of the present invention. In various embodiments, the springis compressed to provide for the insertion of the artificial polymernucleus. In other illustrative embodiments, the artificial nucleus 14 isinjected into the core of the spring 12 after it is implanted.

In illustrative embodiments of the present invention, the spring 12 canbe one of a compression spring, a stacked Belleville washer springs,and/or wave spring. FIG. 9 shows a wave spring in accordance with oneembodiment of the present invention. A wave spring is a coiled flatwire, or multiple flat wires, with waves. Wave springs are superior tocoil springs in certain applications because they provide loweroperating heights while supporting the same loads. As shown in FIGS. 10a and 10 b, wave springs are typically stiffer than conventional coilsprings and therefore have a reduced operating height as compared tocoil springs. Thus, wave springs require less space between and withinthe vertebrae. The wave spring not only provides for space savings, butalso for smaller assemblies that use less materials and thus lowerproduction costs.

Wave springs are a type of compression spring made from an elongatedflat strip of material which is circularly coiled and has a sinusoidalwavepath. Springs are defined by a spring constant which specifies theamount the spring will deflect when a known load is applied to it. Thespring constant of a wave spring is determined by modulus of elasticityof the material, the radial wall thickness, the mean diameter, thenumber of waves per turn, the initial height of the spring, and thethickness of the material. Varying these parameters allows a user totune a spring for a particular application, such as to match the modulusof an intervertebral disc. Additionally, the spring can patient specificie: different dimensions and spring constant for positioning atdifferent levels of the spinal column, and tailored for patients ofdifferent height and weight.

Wave springs can be manufactured using a single turn, crest-to-crest, ornested design. Single turn springs are manufactured from a singlerotation of the sinusoidal wavepath material. Crest-to-crest springs arepre-stacked in series, with the peaks of one layer aligning with thevalleys in the adjacent layer. This stacking decreases the spring rateby a factor relative to the number of turns. Nested springs arepre-stacked in parallel, with the peaks and valleys of the sinusoidalpath aligning on top of each other. This stacking increases the springrate by a factor relative to the number of turns. The above mentionedspring types can additionally be manufactured with flat shims on the topand bottom surfaces of the spring, to more uniformly distribute the loadfrom the spring to the adjacent surfaces.

Wave springs allow for the preservation of anatomical motion. As thespine moves, adjacent vertebrae will eccentrically load the wave springand regions of the spring will experience tension, while the remainingportion will be in a state of compression. Owing to the sinusoidalcoiling, wave springs permit all six degrees of freedom: compression,lateral shear, sagittal shear, flexion/extension, lateral bending, andtorsion. Additionally, because the spring can be made from a strip ofmaterial instead of a wire, wave springs resist shear loads morestrongly than conventional wound springs. This is important formaintaining the stability of the spine, especially during early healing.

Wave springs can act as load bearing devices by compensating foraccumulated tolerances in assemblies and providing end-play take up. Inother words, wave springs exert a force, or “preload” on an assemblymade to the lower end of a tolerance and thereby insure there iscoupling between the components of the assembly. On the other hand, wavesprings also “give” when the components of an assembly are made to thehigh end of the tolerance. The “preload” and “give” properties of thewave spring allow for the spring to support vertebral loads morenaturally while the load conditions on the spine vary.

The spring 12 of the spinal implant 10 can be made from anybiocompatible materials, e.g., shape memory materials, titanium ortitanium alloys, stainless steel, and other similar materials. In oneillustrative embodiment, the springs 12 are made from shape memory metalmaterials such as a Nickel-Titanium alloy (Nitinol) or aTitanium-Niobium alloy. In various illustrative embodiments, the shapememory material offers more “spring” as compared to other alloys becausemany shape memory materials are superelastic. Nitinol alloys are knownfor their superelasticity and thermal shape memory. The term “shapememory effect” is used herein to describe the ability of shape memorymaterials to recover to a predetermined shape upon heating (after havingbeen plastically deformed). The term “superelasticity” refers to theability of the materials to be deformed elastically. For example,Nitinol alloys can be 10 times more elastic as compared to stainlesssteels used in the medical field.

Furthermore, certain shape memory materials, such as Nitinol alloys,follow a non-linear path characterized by a pronounced hysteresis.Nitinol alloys follow the same stress-strain hysteresis curve as bone,thereby making it a very compatible material with bone. Thisrelationship is illustrated in FIG. 11, which shows the stress-straincurve of Nitinol, bone, and stainless steel.

Illustrative embodiments of the spinal implant 10 formed with a shapememory spring 12 include various other advantageous. For example, thesuperelasticity of shape memory materials allows the spring to beinserted into the body with a small compressed profile, makingimplantation a minimally invasive procedure. Once inside the body, thedevices can be released from a constraining means and then can unfold orexpand to a much larger size. A spinal spring formed from shape memorymaterials can be compressed, inserted into the space between thevertebrae, and then allowed to expand into place between the vertebrae.

Another illustrative advantage of a spinal implant 10 formed with ashape memory spring 12 is the shape memory characteristics of shapememory materials. For example, a shape memory material spring with atransition temperature (austenitic start temperature) of 30° C. can becompressed below its transition temperature. The spring will staycompressed until the temperature is raised above 30° C. It will thenexpand to its preset shape. In one illustrative embodiment of thepresent invention, the spring device is kept below its transitiontemperature while being inserted into the body. Once inserted into thespace between the vertebrae, the spring temperature exceeds itstransition temperature due to body heat and the spring expands into itspreset shape.

In various embodiments of the present invention, the spring 12 isconstrained while being inserted into the body in order to preventpremature deployment. Shape memory material springs could be built withtransition temperatures of 40° C. or higher. Such devices would need tobe heated after delivery into the body in order to initiate expansion.

Yet another illustrative advantage of using a shape memory materialspring in a spinal implant is that its loading and unloading curves aresubstantially flat over large strains. This loading curve allowsexemplary embodiments of the spring to apply a constant force. Variousillustrative embodiments of the spinal spring offer a constant stressunder varying loading and unloading conditions (e.g., when a person iswalking or laying down, respectively). Illustrative embodiments of theshape memory material spring will apply a constant force against thevertebrae.

A further illustrative advantage of using a shape memory material springin a spinal implant is its dynamic interference. The dynamicinterference is the long-range nature of shape memory material stresses.For example, unlike an expandable stainless steel spring, self-expandingshape memory material springs expand to their preset shape withoutrecoil. Self-expandable steel springs typically must be over-expanded toachieve a certain diameter as a result of elastic spring-back. Thisspring-back, or loosening, is called acute recoil and is a highlyundesirable feature.

Another illustrative advantage associated with a shape memory materialspring in a spinal implant is its stress hysteresis. In most engineeringmaterials, stress increases with strain in the elastic region when aload is applied to the material and decreases along the same path uponunloading. Some shape memory materials, such as Nitinol, exhibit adistinctly different behavior. FIG. 12 shows the tensile behavior ofstainless steel and Nitinol. As shown in the Nitinol curve, stress firstincreases linearly with strain, up to 1% strain when a load is applied.After a first “yield point”, several percentage points of strain can beaccumulated with only a small amount of stress increase. The end of thisplateau (“loading plateau”) is reached at about 8% strain. After that,there is another linear increase of stress with strain. Unloading fromthe end of the plateau region causes the stress to decrease rapidly,until a lower plateau (“unloading plateau”) is reached. Strain isrecovered in this region with only a small decrease in stress. The lastportion of the deforming strain is finally recovered in a linearfashion. The unloading stress can be as low as 25% of the loadingstress. This looped stress/strain hysteresis is a shared phenomenon withbone, making the two materials compatible, as shown in FIG. 11.

A further illustrative advantage of using a shape memory material springin a spinal implant is that its elasticity is temperature dependent. Inother words, the plateau stresses are strongly temperature dependentabove the transition temperature of the material. As a result,superelastic springs become stiffer when the temperature increases. Thestiffness of a superelastic spring of a given design at a specifictemperature (e.g., body temperature) can be modified to some extent byadjusting the transition temperature of the shape memory material. Thisadjustment can be done by heat treating the material. Lowering thetransition temperature makes the spring stiffer at body temperature.Plotting the loading plateau stress (at a defined strain) versus thedifference of body temperature and transition temperature yields alinear relationship with the stress increasing approximately 4.5 MPa perdegree temperature difference for the most commonly used Nitinol alloys(e.g., alloy with a 50.8% titanium balance to nickel). In illustrativeembodiments, the spring 12 has a tailored stiffness to optimizeperformance by either changing the material composition and/or thethermo-mechanical work applied to the material. To this end, a “sweetspot” of stiffness for torsional stability and matching bone modulus canbe achieved by tuning the chemistry of the material and/or the work/heattreatment regime applied to the material.

Illustrative embodiments of the spinal implant 10 formed with a shapememory material spring can be manufactured in various different manners.In one example, after melting, a Nitinol ingot is forged and rolled intoa bar or a slab at an elevated temperature. Nitinol billets and tubesare extruded at temperatures between 800° C. and 950° C. Such hotworking processes break down the cast structure and improve mechanicalproperties. Next the billets are hot worked into the shape of thespring. An optimal hot working temperature is 800° C. At thistemperature, the Nitinol alloy is easily workable and the surfaceoxidation is limited. Following hot working, the Nitinol spring is coldworked and may be heat-treated to obtain final dimensions with desiredphysical and mechanical properties.

In some cases, cold working of Nitinol is quite challenging because thealloy work-hardens rapidly and thus requires multiple reductions andfrequent inter-pass annealing at 600-800° C. until the final dimensionis obtained. In some illustrative embodiments, the spring 12 is formedfrom round wires. Round wires are produced by a die drawing process.Retaining surface oxide, Nitinol wires can be successfully drawn tosmall sizes.

In other embodiments, the spring 12 is made from rectangular wiresand/or flat wires. FIGS. 13 a and 13 b show a wave spring manufacturedusing flat wires in accordance with one embodiment of the presentinvention. Following a similar reduction schedule, rectangular wires canbe manufactured by drawing round wires, while flat wires are typicallyproduced by cold rolling. In accordance with illustrative embodiments ofthe present invention, as compared to rolled flat wires, the dimensionsof drawn rectangular wires are much more tightly controllable. Forexample, typical tolerances for rolled wire are tighter than for drawnwires.

In some cases, a Nitinol spring is difficult to form at cold workingtemperatures because superelastic Nitinol exhibits significantspring-back when deformed in both cold worked and heat-treated states.Over-deformation of superelastic Nitinol induces martensite andtherefore affects the mechanical and transformation properties. If theNitinol spring is not constrained during heat treatment, the shape ofthe spring will recover partially back to its original configuration.Accordingly, illustrative embodiments of the Nitinol spring arefabricated by using a fixture to hold the spring in a fixed state duringheat treatment. This process can be scaled up to production quantitiesby increasing the number of fixtures and heat treatment capacity. Theformed spring is then placed and constrained in a fixture andsubsequently heat treated to a desired shape with final properties. Inillustrative embodiments of manufacturing a Nitinol wave spring, a flatwire is coiled very tightly at a temperature below the austenite starttemperature and the wave spring is constrained during heat treatment.

In various illustrative embodiments of the present invention, to achieveoptimized properties, materials with 30-40% retained cold work beforeheat treatment should be used. Superelastic Nitinol alloys are typicallyheat treated in the vicinity of 500° C. Lower temperatures in the rangebetween 350° C. and 450° C. are also suitable for Nitinol alloys.Alternatively for Nitinol alloys with greater than 55.5% by weightnickel, good superelasticity and shape memory effect can be obtained bysolution treatment at high temperatures between 600° C. and 900° C. andsubsequent aging at a temperature around 400° C. This aging processinduces precipitation hardening of nickel-rich phases. Thetransformation temperatures are elevated significantly as the matrixcomposition adjusts during aging.

Superelastic shape memory materials have the capability to fully regainthe original shape from a deformed state when the mechanical load thatcauses the deformation is withdrawn. FIG. 14 shows a stress-strain curvefor a superelastic shape memory material in accordance with oneembodiment of the present invention. For some superelastic shape memorymaterials, the recoverable strains can be on the order of 10%. Thisphenomenon, termed as the pseudoelasticity or, superelasticity isdependent on the stress-induced martensitic transformation (SIMT), whichin turn depends on the material's current temperature and the stressapplied to the shape memory material. In one example, a shape memorymaterial that is entirely in the parent (austenitic) phase ismechanically loaded (e.g., material's current temperature is greaterthan the material's austenitic start temperature). Thermodynamicconsiderations indicate that there is a critical stress at which thecrystal phase transformation from austenite to martensite can beinduced. Consequently, the martensite is formed because the appliedstress substitutes for the thermodynamic driving force usually obtainedby cooling the material. The mechanical load, therefore, imparts anoverall deformation to the shape memory material specimen when acritical stress is exceeded. During unloading, because of theinstability of the martensite at this temperature in the absence ofstress, again at a critical stress, the reverse phase transformationstarts from the stress-induced martensitic transformation to the parentphase. When the phase transformation is complete, the shape memorymaterial returns to its parent austenite phase. Therefore, superelasticshape memory material shows a typical hysteresis loop (known aspseudoelasticity or superelasticity) and, if the strain during loadingis fully recoverable, the loop becomes closed.

It should be noted that stress-induced martensitic transformation (orreverse stress-induced martensitic transformation) are marked by areduction of the material's stiffness. Usually the austenite phase has amuch higher Young's modulus in comparison with the martensite phase. Inthe case of a Nitinol wave spring, the more the spring is compressed,the more martensite is induced within the spring and, in turn, thespring becomes less stiff and more elastic. The advantage of thisphenomenon is that, although the Nitinol spring provides more resistanceas its being compressed, the spring also becomes more elastic andyielding because of the phase transformation from austenite tomartensite. In this manner, the Nitinol spring acts as a better shockabsorber for the spine. In contrast, steel springs do not exhibit thisphenomenon and, when compressed to a certain point, the steel springstiffens and could potentially apply an abrupt shock to the spine.Furthermore, the elasticity (or yieldability) of the spring can betailored by modifying the material composition of the spring, modifyingthe work regime applied to the spring, modifying the heat treatmentregime applied to the spring, and/or modifying the design of the spring(e.g., diameter, height, pitch of the spring, or the thickness of thewires).

A wave spring, e.g., made of Nitinol, can be designed to not onlyrestore the height lost by a degenerative disc, but also to correctspinal deformities. As one ages, and one's intervertebral discsdehydrate, the spine can become arched, leading to a “hunched over”appearance. A wave spring, and preferably one made of Nitinol, can beformed in the shape of a wedge, e.g., be shape set to have a wedge shape(for easier insertion), and either by super elasticity or shape memoryeffect apply a restorative force to straighten the vertebral column.

FIGS. 15A-15F show a method for inserting a spinal implant in accordancewith one embodiment of the present invention. FIG. 15A shows a disc andnucleus removal procedure. After removal, FIG. 15B shows the insertionof a spring 12, such as a wave spring. Either before or after insertionof the spring, the entire nucleus may be removed. FIG. 15C shows asizing/diagnostic balloon 18 being inserted into the disc space. FIG.15D shows the balloon being filled with contrast solution. The contrastsolution allows the balloon to be visible in a fluoroscopy. Images ofthe balloon can be taken in order to insure proper positioning andsizing of the balloon within the spring 12. The contrast material andcatheter are then removed. FIG. 15E shows another catheter 20 with aballoon being inserted into the disc space. FIG. 15F shows a mixedpolymer 22 passing through the catheter in liquid form and filling theballoon and the disc space. The polymer cures to form a firm but pliableinner disc, artificial nucleus 14, which is surrounded by the spring 12.

In accordance with illustrative embodiments of the present invention, aspinal implant 10 includes a wave spring 12 with a large cross sectionalarea. The spinal implant mimics the natural disc. The spinal implant 10includes an inner artificial (polymer) nucleus area 14. Furthermore, inillustrative embodiments, the spinal implant includes contour endplateswith teeth (not shown) that couple to the vertebrae.

Illustrative embodiments of the spinal implant 10 include a pliablemodulus of elasticity which contributes to a broad and uniformdistribution of pressure on the vertebral end plates. In variousillustrative embodiments, the spinal implant 10 fills the entire volumeof the disc cavity and contours the endplates, thus enhancing the loadsharing between the annulus and the implant 10. Such a designfacilitates stability and functional performance after implantation. Inadditional or alternative embodiments, the implantation of large volumespinal implant through a small annulotomy prevents migration of theimplant. Furthermore, illustrative embodiments of the present inventioncontribute to prevent endplate reaction with the bone surface andthereby prevent loosening, in contrast to other disc and nucleusreplacement devices reported clinically.

In some embodiments, the spiral implant may include a polymer sheath(not shown) surrounding the spring 12 and the nucleus 14 in order toprevent bone in-growth.

The embodiments of the invention described above are intended to bemerely exemplary; numerous variations and modifications will be apparentto those skilled in the art. All such variations and modifications areintended to be within the scope of the present invention.

What is claimed is:
 1. A spinal implant comprising: a coiled wave springconfigured to surround a nucleus, wherein the wave spring is formed withat least one wire having a sinusoidal shape and made of a shape memorymaterial, and the shape memory material is tailored to achieve astress-induced martensitic transformation when a critical stress isexceeded.
 2. The spinal implant according to claim 1, wherein the shapememory material is further trained to expand to a preset shape when thewave spring temperature exceeds its transition temperature.
 3. Thespinal implant according to claim 1, wherein the wave spring is wedgeshaped.
 4. The spinal implant according to claim 1, wherein the shapememory material is selected from the group consisting of Nitinol, aTitanium-Niobium alloy, and combinations thereof.
 5. The spinal implantaccording to claim 1, wherein the wave spring is formed with one or moreflat wires.
 6. The spinal implant according to claim 1, wherein the wavespring is formed with one or more rectangular wires.
 7. The spinalimplant according to claim 1, further comprising: an artificial nucleusconfigured to simulate a disc nucleus, wherein the wave spring surroundsthe artificial nucleus.
 8. The spinal implant according to claim 7,wherein the artificial nucleus is made from a polymer material or ahydro-gel material.
 9. The spinal implant according to claim 7, whereinthe artificial nucleus is a wave spring.
 10. A method of forming aspinal implant, the method comprising: forming a coiled wave spring, thewave spring having at least one wire with a sinusoidal shape and made ofa shape memory material, and the shape memory material is tailored toachieve a stress-induced martensitic transformation when a criticalstress is exceeded; and configuring the wave spring to surrounding anucleus.
 11. The method according to claim 10, wherein the wave springis wedge shaped.
 12. The method according to claim 10, wherein the shapememory material is selected from the group consisting of Nitinol, aTitanium-Niobium alloy, and combinations thereof.
 13. The methodaccording to claim 10, wherein the wave spring is formed with one ormore flat wires or rectangular wires.
 14. The method according to claim10, further comprising forming an artificial nucleus configured tosimulate a disc nucleus, wherein the wave spring surrounds theartificial nucleus.
 15. The method according to claim 14, wherein theartificial nucleus is formed from a polymer material or a hydro-gelmaterial.
 16. The method according to claim 14, wherein the artificialnucleus is formed from a wave spring.
 17. The method according to claim10, wherein forming the coiled wave spring further comprises: providingthe shape memory material with about 30-40% cold work; coiling the wavespring; and subsequently age heat treating the shape memory materialafter coiling the wave spring.
 18. The method according to claim 10,wherein the shape memory material is further trained to expand to apreset shape when the wave spring temperature exceeds its transitiontemperature.
 19. A method of implanting a spinal implant, the methodcomprising: inserting a coiled wave spring into an intervertebral space,wherein the wave spring is formed with at least one wire having asinusoidal shape and made of a shape memory material, and the shapememory material is tailored to achieve a stress-induced martensitictransformation when a critical stress is exceeded; and introducing anucleus material into an interior area of the wave spring, the nucleusmaterial configured to simulate a disc nucleus.
 20. The method accordingto claim 19, wherein the shape memory material is further trained toexpand to a preset shape when the wave spring temperature exceeds itstransition temperature.